As a result of the transition from analogue to digital TV transmissions, for example in the 470-862 MHz frequency band, certain portions of the spectrum may no longer be used for TV transmissions, though the exact frequency of unused spectrum may vary from location to location. These unused portions of spectrum may be referred to as TV White Space (TVWS). The Federal Communications Commission (FCC) has opened up these TVWS frequencies for a variety of unlicensed uses. These frequencies may be exploited by secondary users for any radio communication, and such frequencies may be used by the LTE and other cellular technologies. Long Term Evolution (LTE) may also use other unlicensed bands such as Industrial, Scientific and Medical (ISM) band.
When an LTE system operates in a license exempt (LE) band, the same spectrum may be shared with other secondary users using a radio access technology different from LTE. Coexistence with other secondary users may result in the LTE system operating in an environment with higher interference signals, not typically present in licensed band. For example, an LTE system may coexist with a Wi-Fi system. Some of the Wi-Fi systems may use a backoff threshold of −62 dBm. Therefore, unless the Wi-Fi system is very close to the eNB operating on the supplementary component carrier (SuppCC), interference from the Wi-Fi system may need to be overcome by Physical Datalink Control Channel (PDCCH). Although cross-carrier scheduling may be used to schedule resources in the SuppCC from PDCCH on the licensed band, PDCCH congestion may occur even for a small number of SuppCCs.
When the PDCCH is signaled on the SuppCC, it may experience interference from primary users operating in adjacent channels. It may also experience interference from secondary users sharing the same channel. In addition, a simple increase in the signal to noise ratio (SNR) may not be enough to address the PDCCH performance.
For reliable PDCCH operation, cross-carrier scheduling may be used from the primary cell to allocate supplementary cell resources. However, PDCCH congestion/blocking may occur within the system. The use of supplementary cell to carry PDCCH signals may be provided.